The field of the invention is optical service cable, sometimes called drop cable.
Bell Communications Research issued its technical reference TR-TSY-000843 in January of 1989 titled "Generic Requirements for Optical and Optical/Metallic Buried Service Cable". As the subject of the invention is a dielectric service cable, any requirements for metallic members in an optical/metallic buried service cable will not be set out herein.
As stated in the technical reference, the environment of buried service cable is one of the most severe in the network. Such cables run from a service access point to individual subscribers, which may be homes or businesses. Some portions of the cable may be above ground, while other portions are buried only to a shallow depth. Because the cable will be attached to a building network, fire retardancy and flexibility are requirements. The cable will typically be bent to a two inch diameter and slack is stored in coils of small diameter.
The stated requirements for non-armored all dielectric cables state that optical fibers are contained in the core which is enclosed by a jacket. Water-blocking materials are to be used within the core and between the core and the jacket. A jacket slitting cord is also required. FIG. 2--2 of the technical reference discloses two optical fibers and filling compound within a buffer tube. The buffer tube is coated by a flame retardant jacket having six strength members embedded therein at approximately equal angles of separation from each other. Other cable requirements and tests are included, such as cable bend test and flammability testing. It is further stated that the outer jacket shall be black plasticized polyvinyl chloride (PVC).
In practice, the requirements for buried service cable have proven to be challenging.
AT&T produced a dielectric drop cable in 1989 which featured two buffered optical fibers and a filling compound in a nylon tube. A PVC jacket was directly extruded over three impregnated fiberglass strands and two water swellable threads. With so few strength members, it is believed that fiber strain could be equivalent to cable strain under certain conditions. In addition, it is believed that this cable might fail the required flame test.
Another early dielectric drop cable was produced by Siecor in 1989. This cable included tensile members, but no antibuckling members, between the core buffer tube and the outer jacket. As a result, this cable had a flexural modulus of elasticity of less than 50 MPa, and ultimately proved to be subject to kinking.
A later AT&T dielectric drop cable was produced in 1991. The cable contained two fight buffered fibers and a filling compound in a nylon buffer tube. A PVC outer jacket was directly extruded over two groups of two impregnated fiberglass strands located 180 degrees apart between the buffer tube and the outer jacket. A water swellable yarn was provided with each pair of strength elements. While this cable was far stronger than the previous cables, it was extremely stiff and difficult to handle, having a flexural modulus of elasticity of greater than 400 MPa. Thus, it is believed that this cable could be bent into two inch loops with only the greatest difficulty, if at all.
Northern Telecom has also introduced a two fiber drop cable stated to meet the demands of the technical reference. This cable has a metallic sheath, however. An unbroken layer of glass strength members is laid over the buffer tube and a water swellable tape is wrapped around the strength members to enhance water blocking performance. A water blocking gel along with two tight buffered optical fibers are contained in the buffer tube.
It is believed that the cables described above have not adequately met the needs of the industry for a dielectric drop cable which meets the requirements of the technical reference, is not too stiff, yet does not kink. These requirements are met by the cable according to the invention.